http://www.themodernantiquarian.com/post/32067
There's some stuff at the bottom of this photo (is it of the stone next to the entrance?). Three (maybe four) small holes (one looks countersunk), two with circles round them and something that looks like a 1 or a 4 amongst them. All modern looking.
Do they help clarify what the groove might be? Or do they just add to the mystery?